Alloy sheet for shadow mask and method for manufacturing thereof

Abstract: An alloy sheet containing Fe, Ni and Cr has an average austenite grain size of 15 to 45 µm and a degree of mixed grain for austenite grain size of 50% or less; said alloy sheet has a gathering degree of {331} plane on a surface of said alloy sheet being 35% or less, that of {210} plane 20% or less and that of {211} plane 20% or less; said degree of mixed grain is expressed by an eqution of (|0.5 Dmax - D|/D) x 100 (%) , where D is an average austenite grain size, and Dmax is a maximum austenite grain size in said alloy sheet. A method for manufacturing an alloy sheet for a shadow mask comprising the steps of (a) hot-rolling, (b) annealing of hot-rolled sheet, (c) cold-rolling, (d) recrystallization annealing, (e) finish cold-rolling, (f) stress relief annealing and (g) annealing before press-forming.

Description: Background of the Invention Field of the Invention The present invention relates to an alloy sheet for making a shadow mask used for color cathode ray tube and method for manufacturing thereof, in particular to an alloy sheet containing Fe, Ni and Cr having high press-formability and method for manufacturing thereof. Description of the Related Arts Recent up-grading trend of color television toward high definition TV has employed Fe-Ni alloy containing 34 to 38wt.% Ni as the alloy for making a shadow mask to suppress color-phase shift. Compared with low carbon steel which has long been used as a shadow mask material, conventional Fe-Ni alloy has considerably lower thermal expansion coefficient. Accordingly, a shadow mask made of conventional Fe-Ni alloy raises no problem of color-phase shift coming from the thermal expansion of shadow mask even when an electron beam heats the shadow mask. Common practice of making the alloy sheet for shadow mask includes the following steps. An alloy ingot is prepared by continuous casting process or ingot-making process. The alloy ingot is subjected to slabbing, hot-rolling, cold-rolling, and annealing to produce an alloy sheet. The alloy sheet for making a shadow mask is then processed usually in the following steps to form shadow mask. (1) The alloy sheet is photo-etched to form passage-holes for the electron beam on the alloy sheet for making a shadow mask. The alloy sheet for making a shadow mask perforated by etching is hereinafter referred to as "flat mask". (2) The flat mask is subjected to annealing. (3) The annealed flat mask is pressed into a curved shape of cathode ray tube. (4) The press-formed flat mask is assembled to a shadow mask which is then subjected to blackening treatment. However, the above mentioned conventional Fe-Ni alloy has higher strength and plane anisotropy as mechanical property than conventional low carbon steel. Accordingly, alloy sheet for shadow mask prepared by the process including first cold-rolling, recrystallization annealing and finish cold-rolling, which is annealed before press-forming after perforated by etching, develops problems such as poor shape-fix ability, cracking on the alloy sheet and blurred periphery of the pierced hole during press-forming, which is a significant disadvantage of manufacturing a cathode ray tube. In addition, the above described Fe-Ni alloy gathers rust easily, which reduces production yield druing the manufacturing process of shadow mask. The prior art was proposed in JP-A-3-267320 (the term JP-A- referred to herein signifies unexamined Japanese patent publication), where a method to decrease the strength of the conventional Fe-Ni alloy and solve the problems is provided. Hereafter, said art is referred to as prior art 1. The prior art 1 employs first cold-rolling, recrystallization annealing, finish cold-rolling and softening annealing. The finish cold-rolling is performed at a reduction ratio of 5 to 20%, and the temperature of the softening annealing is less than 800°C. The prior art 1 produces a sheet having sufficiently low strength to give good press-formability with the 0.2% proof stress of 9.5 kgf/mm 2 (10 kgf/mm 2 or less) at 200°C. The prior art 2 was proposed in JP-B-64-52024, where a method to reduce a plane anisotropy as mechanical property is provided. To produce a plate for shadow mask having the less plane anisotropy of elastic coefficient, the prior art 2 employs the process including cold-rolling followed by recrystallization annealing repeated twice or more, and cold-rolling to increase hardness. In that process, finish cold-rolling before finish recrystallization annealing is performed at the reduction ratio of 40 to 80%. The plate for shadow mask has excellent uniform formability during press-forming resulting in a small deformation of etched-hole and free form the irregular gloss and stringer defect when the plate is etched, annealed and press-formed. However, the prior art 1 does not satisfy the quality required to perform a favorable warm press-forming, though the strength is lowered to the level suitable for press-forming under the above described annealing condition. The alloy sheet for shadow mask prepared by the prior art was found to gall a die and to generate cracks at the edge of shadow masks. Furthermore, the above described alloy sheet for making a shadow mask often develops a quality problem of blurred periphery of pierced hole after press-forming, as the plane anisotropy of the alloy is large. In the prior art 2, plane anisotropy of elastic coefficient of the plate is small, and the blurred periphery of pierced hole according to the deformation of the hole during the press-forming is not obserbed. Still, the prior art 2 induces cracks at the edge of shadow mask during press-forming and does not improve corrosion resistance of Fe-Ni alloy. EP-A-0 552 800 discloses an alloy sheet for a shadow mask in which the degrees of the planes {211}, {210} and {331} are controlled. Not disclosed is the degree of mixed grain for austenite grains or the average austenite grain size. Present color televisions request severer quality specification on color-phase shift because the color picture tubes direct to brighter and more flat face than ever. The cathode ray tubes using the shadow masks prepared by the prior arts mentioned above give partial color-phase shift under electron beam irradiation. Summary of the Invention The object of the present invention is to provide an alloy sheet for making a shadow mask having excellent corrosion resistance and high press-formability, which developes no crack nor blurred periphery of pierced hole during press-forming and does not generate a color phase shift when used for cathode ray tubes and method for manufacturing thereof. To achieve the object, the present invention provides an alloy sheet as defined in claims 1 and 4. Preferred embodiments are given in the dependent claims. The present invention also provides a method for manufacturing an alloy sheet defined in claims 9 and 13. Preferred embodiments are given in the dependent claims. Brief Description of the Drawings Fig. 1 is a graph showing a relation among crack generation during press-forming, gathering degree of {211} plane, and average austenite grain size after the annealing before press-forming according to the present invention; Fig. 2 is a graph showing a relation between the blurred periphery of pierced hole at press-forming and degree of mixed grain for austenite grains according to the present invention; Fig. 3 is a graph showing a relation between the degree of mixed grain for austenite grains and the reduction ratio of first cold-rolling where the process includes only one cold-rolling, according to the present invention; Fig. 4 is a graph showing a relation among the degree of mixed grain for austenite grains, reduction ratio of the first cold-rolling and reduction ratio of the second cold-rolling where the process includes two cold-rollings, according to the present invention; and Fig. 5 is a graph showing the range of the condition of annealing before press-forming according to the present invention. Description of the Preferred Embodiments The inventors made extensive study to develop a Fe-Ni alloy sheet and Fe-Ni-Cr alloy sheet for shadow mask having high press-formability and suppressing partial color phase shift and found that the desired press-formability is obtained while suppressing the above mentioned color phase shift by adjusting the chemical composition, austenite grain size, degree of mixed grains for austenite grains, and crystal orientation of the Fe-Ni alloy sheet for making a shadow mask within a specified range. Presence of B and O within a specified range enhances growth of crystal grains during the annealing before press-forming. The growth of crystal grains yields the austenite grain having specified size, which gives shape fix ability during press-forming. Also the presence of Si and N within a specified range supresses the galling of dies and improves the fitness to die during press-forming. The control of gathering degree of {211} plane on the alloy sheet after annealing before press-forming within a specified range suppresses the generation of crack on material during press-forming. By adjusting the degree of mixed grain for austenite grains after the annealing before press-forming within a specified range, the generation of blurred periphery of pierced hole during press-forming is suppressed. Also by adjusting the gathering degrees of {210} plane and of {331} plane of the alloy sheet after annealing before press-forming within a specified range, the above described color-phase shift can be suppressed. In the manufacturing process of the alloy of the present invention, the hot-rolled strip is subjected to hot-rolled sheet annealing at a specific temperature before cold-rolling. Both cold-rolling and finish cold-rolling control their reduction ratio, and the annealing before press-forming controls the condition within each specified range. The average austenite grain size and gathering degree of {331}, {210} and {211} plane on the surface of alloy sheet are adjusted within specified range. To maintain the degree of mixed grain for austenite grains in the alloy sheet after the annealing before press-forming within a specified range, once or twice of cold-rolling after the annealing of hot-rolled sheet are conducted under a reduction ratio within a specified range. This invention has been derived based on the findings described above. The reason of specifying the chemical composition, austenite grain size and degree of mixed grain for austenite grains after annealing before press-forming, and the gathering degrees of {331}, {210}, and {211} plane of the Fe-Ni alloy sheet and Fe-Ni-Co-Cr alloy sheet surface for making shadow mask of the present invention within the range above described is clarified in the following. (1) Nickel: To prevent color-phase shift, the Fe-Ni alloy sheet for making a shadow mask is necessary to have the upper limit of average thermal expansion coefficient as approximately 2.0 x 10 -6 /°C in the temperature range of 30 to 100°C. The thermal expansion coefficient depends on the content of Ni in the alloy sheet. The Ni content which satisfies the above limitation of average thermal expansion coefficient is 34 to 38 wt.%. Consequently, the Ni content should be limited to 34 to 38wt.%. More preferably, the Ni content to decrease average thermal expansion coefficient is 35 to 37 wt.%, and most preferably 35.5 to 36.5 wt.%. When the alloy includes 0.001 to 1.0 wt.% cobalt, Ni content to satisfy the upper limit of the average thermal expansion coefficient is 34 to 38 wt.%, and most preferably in a range of 35 to 38 wt.%. When the alloy includes over 1.0 to 7wt.% Co, the range of Ni content to satisfy the above described condition of average thermal expansion coefficient is 27 to 38 wt.%, and the average thermal expansion coefficients of Fe-Ni-Co-Cr alloy and Fe-Ni-Cr aloy are further reduced by limiting Ni content to 30 to 33 wt.% and Co to 3 to 6 wt.%. Chromium improves the corrosion resistance of alloy, but increases thermal expansion coefficient. Chromium content of less than 0.05% gives no effect of improvement in corrosion resistance. On the other hand, when the Cr content exceeds 3wt.%, the alloy can not provide the average thermal expansion coefficient specified by the present invention. Therefore, the lower limit and the upper limit of Cr content are specified as 0.05 wt.% and 3.0 wt.%, respectively. According to this invention, the average austenite grain size which is requested from the reason of improving shape fix ability, suppressing the generation of crack on alloy sheet during press-forming and of preventing the blurred periphery of pierced hole after press-forming is 15 to 45 µm for warm press-forming. Grain size of less than 15 µm gives poor shape fix ability and generates cracks on alloy sheet. Grain size above 45 µm, however, generates cracks and blurred periphery of pierced hole after press-forming. Consequently, the average austenite grain size is specified as 15 to 45 µm. To suppress the generation of crack on material, the control of gathering degree of {211} plane on alloy sheet within a specified range while maintaining the above described average austenite grain size is essential as described later. To enhance the grain growth under the condition of annealing before press-forming of this invention, the control of O and B content of specified level or less is necessary. To improve the fitness to dies during press-forming, the control of Si and N content of specified level or less is necessary. The following is the description of the content of such elements. (2) Oxygen: Oxygen is one of the inevitable impurities. Increased content of O increases the non-metallic oxide inclusion within the alloy, which inclusion suppresses the growth of crystal grains during the annealing before press-forming, particularly in the temperature range of 740 to 900°C for 40min. or less. If the content of O exceeds 0.0030 wt.%, then the growth of crystal grains described above is considerably suppressed, and no austenite grain size aimed in this invention is obtained. Consequently, the upper limit of O content is specified to 0.0030% and the lower limit is specified to 0.0001%. (3) Boron: Boron enhances the hot-workability of the alloy. Excess amount of B, however, induces the segregation of B at boundary of recrystallized grains formed during the annealing before press-forming, which inhibits the free migration of grain boundaries and results in the suppression of grain growth and the dissatisfaction of austenite grain size after the annealing before press-forming. In particular, under the annealing condition before press-forming, which is specified in this invention, the suppression action against the grain growth is strong and the action does not uniformly affect on all grains. As a result, a severe mixed grain structure appears accompanied with irregular elongation of material during press-forming, which induces blurred periphery of pierced hole. Boron also increases the gathering degree of {211} plane after annealing, which causes the crack on a skirt of material. Boron content above 0.0030 wt.% significantly enhances the suppression of grain growth, and the austenite grain size aimed in the present invention can not be obtained. Also the blurred periphery of pierced hole during press-forming occurs, and the gathering degree of {211} plane exceeds the upper limit specified in the present invention. Based on these findings, the upper limit of B content is defined as 0.0030 wt.%. (4) Silicon: Silicon is used as the deoxidizer during ingot-making of the alloy. When the Si content exceeds 0.10 wt.%, an oxide film of Si is formed on the surface of alloy during the annealing before press-forming. The oxide film degrades the fitness to dies during press-forming and results in the galling of dies with alloy sheet. Consequently, the upper limit of Si content is specified as 0.10 wt.%. Less Si content further improves the fitness of dies and alloy sheet. The lower limit is specified to 0.001%. (5) Nitrogen: Nitrogen is an element unavoidably entering into the alloy during ingot-making process. Nitrogen content of more than 0.0030 wt.% induces the concentration of N on the surface of alloy during the annealing before press-forming. The concentrated N on the surface of alloy degrades the fitness to dies during press-forming and induces galling of dies with the alloy sheet. Consequently, the upper limit of N content is specified as 0.0030 wt.% and the lower limit is specified to 0.0001%. An alloy for shadow mask of the present invention contains specific amount of O, B, Si, and N in its Fe-Ni-Cr and Fe-Ni-Co-Cr basic composition, and has an average austenite grain size of 15 to 45 µm after annealing before press-forming, and has degree of mixed grain for austenite grains of 50% or less, and has 20% or less, 35% or less, 20% or less of the gathering degree of {211}, {331}, {210} plane, respectively. Most preferably, the composition further contains 0.0001 to 0.0040 wt.% C, 0.001 to 0.35 wt.% Mn, and 2.0 ppm or less H. As described above, the control of chemical composition and of average austenite grain size after annealing before press-forming within the range specified in the present invention suppresses the galling of dies with alloy during press-forming and gives a superior shape fix ability. However, there remains the problem of crack generation on press-formed material. To cope with the problem, the inventors studied the relation between the crack generation and the crystal grain orientation during press-forming by changing the crystal grain orientation of the alloy sheet in various directions using the alloy sheets having chemical composition and average austenite grain size in the range specified in the present invention, and found that an effective condition to suppress the crack generation on the alloy material is to control the gathering degree of {211} plane at or less than a specific value, as well as to control the average austenite grain size after the annealing before press-forming at or less than a specific level. Fig. 1 shows the relation among crack generation on alloy sheet during press-forming, gathering degree of {211} plane, and average austenite grain size for alloy sheet having chemical composition specified in the present invention. The gathering degree of {211} plane is determined from the relative X-ray diffraction intensity ratio of (422) diffraction plane of alloy sheet after the annealing before press-forming divided by the sum of relative X-ray diffraction intensity ratio of (111), (200), (220), (311), (331), (420), and (422) diffraction planes. The measurement of gathering degree of {211} plane was carried by measuring the diffraction of {422} plane which has equivalent orientation with {211} plane. The relative X-ray diffraction intensity ratio is defined as the value of X-ray diffraction intensity observed on each diffraction plane divided by the theoretical X-ray diffraction intensity of that diffraction plane. For example, the relative X-ray diffraction intensity ratio of (111) diffraction plane is determined from the X-ray diffraction intensity of (111) diffraction plane divided by the theoretical X-ray diffraction intensity of (111) diffraction plane. The measurement of the gathering degree of {331} and {210} plane, which will be described in detail later, was carried by measuring the relative X-ray diffraction intensity ratio of (331) diffraction plane and (420) diffraction plane (which have the equivalent factor with {211} plane) divided by the sum of relative X-ray intensity ratio of seven diffraction planes from (111) to (422) described above, respectively. Fig. 1 clearly shows that the case where average austenite grain size is 15 to 45 µm and where the gathering degree of {211} plane is 20% or less does not generate crack on alloy sheet during press-forming and does not induce blurred periphery of the pierced hole, which fact indicates the excellent effect of the present invention. Based on the findings, the invention specifies the gathering degree of {211} plane as 20% or less as the condition to suppress crack generation on the alloy sheet. Adding to the measures above described, the control of degree of mixed grain for austenite grains after the annealing before press-forming is necessary to prevent the generation of blurred periphery of the pierced hole during press-forming. Fig. 2 shows the relation between the frequency of blurred periphery of the pierced hole and degree of mixed grain for austenite grains after press-forming using an alloy sheet having chemical composition, average austenite grain size and the gathering degrees of {331}, {210}, and {211} plane within the range specified in the present invention. The figure indicates that the degree of mixed grain for austenite grains more than 50% increases the frequency of blurred periphery of pierced hole. Consequently, the degree of mixed grain for austenite grains to suppress generation of blurred periphery of the pierced hole after press-forming is specified as 50% or less. As detailed above, with the specification of O, B, Si, and N content and of average austenite grain size and the degree of mixed grain after annealing before press-forming and of gathering degree of {211} plane, the press-formability aimed in the present invention becomes sufficient. To suppress partial color-phase shift, the control of gathering degrees of {331} plane and {210} plane after annealing before press-forming is important, which is described before. When the gathering degrees of {331} and {210} plane exceeds 35% and 20% after annealing before press-forming, respectively, partial color-phase shift will occur. Consequently, this invention specifies the degrees of {331} plane and {210} plane as 35% or less and 20% or less, respectively. The method to keep the gathering degrees of {331}, {210}, and {211} plane after annealing before press-forming at or less than 35%, 16%, and 20% is to adopt the alloy sheet manufacturing condition which prevent aggregation of {331}, {210}, and {211} plane during the steps of solidification, hot-rolling, cold-rolling, and annealing of this alloy sheet as far as possible. For example, when the alloy is manufactured from a hot-rolled strip prepared from slabs or continuous casting slabs through ingot-making and slabbing, the hot-rolled strip is subjected to annealing of hot-rolled sheet, cold-rolling, recrystallization annealing, finish cold-rolling, stress relief annealing, annealing before press-forming, press-forming, and blackening. In the process, adequate annealing of hot-rolled sheet after hot-rolling is effective to prevent gathering of {331}, {210}, and {211} plane. Particularly when the temperature of the annealing of hot-rolled sheet is adequately selected in the range of 810 to 890°C, the gathering degree of {331}, {210}, and {211} plane can be kept at or less than the level specified in this invention. Consequently, the present invention specifies the annealing temperature of hot-rolled sheet as 810 to 890°C which provides the gathering degree of {331} plane of 35% or less, degree of {210} plane of 20% or less, and degree of {211} plane of 20% or less. The effect of annealing of hot-rolled sheet in the present invention is performed when the hot-rolled alloy strip is fully recrystallized before the annealing of hot-rolled sheet. To obtain the satisfactory gathering degrees of {331}, {210}, and {211} plane being focused on in the present invention, the uniform heat treatment of the slab after slabbing is not preferable. For example, when a uniform heat treatment is carried at a temperature of 1200°C or more for a period of 10 hours or more, at least one of the gathering degrees of {331}, {210}, and {211} plane exceeds the range specified in the present invention. Therefore, such a uniform heat treatment must be avoided. In addition, when the above described hot-rolled strip is used in the manufacturing process, the optimization of all conditions in the cold-rolling, annealing, finish cold-rolling, stress-relief annealing, and annealing before press-forming are required to assure the gathering degrees of {331}, {210}, and {211} plane and the degree of mixed grain for austenite grains to be in the range specified in this invention. After the annealing of hot-rolled sheet, the optimization of conditions of cold-rolling and annealing is important to control the gathering degree of mixed grain for austenite grains after annealing before press-forming. An alloy of the present invention was subjected to annealing of hot-rolled sheet and single cycle of cold-rolling and annealing. Fig. 3 shows the relation between the reduction ratio of cold-rolling and the degree of mixed grain for austenite grains after annealing before press-forming. In concrete terms, this is the graph showing the relation between the degree of mixed grain for austenite grains and the reduction ratio of first cold-rolling of the alloy sheet prepared from hot-rolled strip which was subjected to the process including the annealing of hot-rolled sheet in the temperature range of 810 to 890°C, first cold-rolling at the reduction ratio of 73 to 97%, recrystallization, finish cold-rolling at the reduction ratio of 14 to 29%, and stress-relief annealing in the temperature range of 450 to 540°C for 0.5 to 300 sec., followed by the annealing in the temperature range of 740 to 900°C for 2 to 40min. before press-forming. Fig. 3 indicates that the reduction ratio of cold-rolling of 81 to 94% gives the degree of mixed grain for austenite grains of 50% or less when the process of intermediate cold-rolling and annealing are conducted only once. On the other hand, the reduction ratio of cold-rolling of less than 81% or more than 94% gives the degree of mixed grain for austenite grains of more than 50%. Consequently, the present invention specifies the reduction ratio of cold-rolling as 81 to 94% to secure the degree of mixed grain for austenite grains at or below 50% for single cycle of cold-rolling and annealing. Fig. 4 is a graph showing a relation between the reduction ratio of cold-rolling and a degree of mixed grain for austenite grains when the process of intermediate cold-rolling and annealing is performed twice. In concrete terms, this is the graph showing the relation between the degree of mixed grain for austenite grains and the reduction ratios of first cold-rolling and second cold-rolling of the alloy sheet prepared from hot-rolled strip which was subjected to the process including the annealing of hot-rolled sheet in the temperature range of 810 to 890°C, first cold-rolling at the reduction ratio of 35 to 60%, recrystallization annealing, second cold-rolling at the reduction ratio of 75 to 97%, recrystallization annealing, finish cold-rolling at the reduction ratio of 14 to 29%, and stress relief annealing in the temperature range of 450 to 540°C for 0.5 to 300 sec., followed by annealing in the temperature range of 740 to 900°C for 2 to 40 min. which annealing time (T) satisfying the equation of: T ≥ - 123logt + 937. As shown in Fig. 4, the degree of mixed grain for austenite grains gives satisfactory value when the reduction ratio of second cold-rolling is 81 to 94 % and the reduction ratio of first cold-rolling is 40 to 55%. From the reasons described above, the present invention specifies the reduction ratio of first cold-rolling as 40 to 55% and that of second cold-rolling as 81 to 94% for the case of two cycles of cold-rolling and annealing. The recrystallization annealing after first and second cold-rolling is preferably carried at 810 to 840°C for 0.5 to 3min. Even when the annealing temperature exceeds the recrystallization temperature, the annealing at the temperature of 810°C or less induces the formation of mixed grains, and the degree of mixed grain for austenite grains increases after the annealing before press-forming. Even if the temperature is kept in the range of 810 to 840°C, in the case of annealing of below 0.5min or over 3min., the texture shows a mixed grain structure. The both latter two cases are unfavorable because of the increase of degree of mixed grain for austenite grains after annealing before press-forming. When the conditions of cold-rolling and annealing which are described above are adopted, the gathering degrees of {331}, {210}, and {211} plane becomes 35% or less, 20% or less, and 20% or less, respectively. As for the reduction ratio of finish cold-rolling of 14 to 29%, when the conditions of chemical composition, cold-rolling, annealing, and annealing before press-forming are selected in the range specified in this invention, the resulted characteristics are: 15 - 45 µm of average austenite grain size; 50% or less of degree of mixed grain for austenite grains; 35% or less of gathering degree of {331} plane; 20% or less of gathering degree of {210} plane; and 20% or less of gathering degree of {211} plane, after the annealing before press-forming. In the case that the reduction ratio of finish cold-rolling is less than 14% or more than 29%, at least one of the above described characteristics which are the feature of the present invention becomes out of the scope of the present invention. Consequently, the reduction ratio of finish cold-rolling is specified as 14 to 29%. According to the present invention, the optimization of the condition of annealing before press-forming is also important to obtain the value of average austenite grain size, degree of mixed grain for austenite grains. Fig. 5 shows the relation among average austenite grain size, degree of mixed grain for austenite grains, gathering degrees of {331}, {210}, and {211} plane after annealing before press-forming, and temperature (T, °C) and time (t , min.) of annealing before press-forming under the conditions of hot-rolled sheet annealing, cold-rolling and annealing, and the reduction ratio of finish cold-rolling within the range specified in the present invention. As clearly shown in Fig. 5, even if all the conditions other than the condition of annealing before press-forming are within the range of the present invention, when the annealing temperature follows the relation of T < - 123 log t + 937, then the average austenite grain size is less than 15 µm and the gathering degree of {211} plane is more than 20%, which values are not suitable for the present invention. When annealing temperature (T) exceeds 900°C, the average austenite grain size exceeds 45 µm and gathering degree of {211} plane exceeds 20%, the values are inadequate. Furthermore, when annealing time (t) exceeds 40min., at least one of the gathering degrees of {211}, {331}, and {210} plane exceeds the specified range of the present invention, which is not suitable. Consequently, the condition to obtain satisfactory values of average austenite grain size, degree of mixed grain for austenite grains, and gathering degrees of {331}, {210}, and {211} plane is specified in the present invention as T : 740 to 900°C, t : 2 to 40 min., and T ≥ -123 log t + 937. The stress relief annealing in the present invention is important to control the gathering degrees of {331}, {210}, and {211} plane in the succeeding annealing before press-forming. The stress relief annealing should be conducted at 450 to 540°C for 0.5 to 300 sec. to demonstrate the enough effect aimed in the present invention. There are other method to limit the gathering degrees of {331}, {210}, and {211} plane on the alloy sheet after the annealing before press-forming within the range specified in the present invention. Examples of these methods are quenching solidification and texture structure control through the control of recrystallization during hot-working. The annealing before press-forming in the present invention may be applied before photo-etching. In that case, desired photo-etching quality is ensured only when the condition of annealing before press-forming is kept within the range specified in this invention. Example The present invention is described in more detail in the following referring to examples. Example-1 Slabs of No. 1 to No. 21 having chemical compositions listed in Table 1 were prepared by continuous casting a molten steel obtained by ladle refining. The chemical composition other than hydrogen is expressed by wt.% and the hydrogen content is expressed by p.p.m (parts per million) in Table 1. The continuous cast slabs were subjected to scarfing, then heated at 1100°C for 3 hrs in a heating furnace and hot-rolled to obtain hot-rolled strips. The hot-rolled strips were subjected to a process including annealing of hot-rolled sheet at 860°C, first cold-rolling at the reduction ratio of 93%, recrystallization annealing at 810°C for 1 min. The process further includes finish cold-rolling at the reduction ratio of 21%, stress-relief annealing at 530°C for 1 min. to obtain alloy sheets having 0.25mm thickness. The alloy sheets obtained by the above described method were put to various experiments described below. The materials of No. 1 to No. 21 correspond to the alloys of No. 1 to No. 21. Said hot-rolled strips which were fully recrystallized after hot-rolling were selected. The alloy sheets of materials No. 1 through No. 3, No. 5 through No. 21 were formed into flat masks by etching and then they were subjected to annealing before press-forming under the condition specified in Table 2 and to press-forming. The press-formed sheets were tested to determine the shape fix ability, fitness to dies, crack generation on material, and frequency of blurred periphery of pierced hole based on the evaluation criteria given in Table 4. The materials were investigated for the corrosion resistance after stress relief annealing based on the evaluation criteria given in Table 4. Material No. Temperature of annealing of hot-rolled sheet (°C) Annealing condition before press-forming Temperature: T (°C) Time: t (min.) 1 860 830 30 2 860 850 40 3 860 870 15 4 860 880 3 5 860 750 40 6 860 790 25 7 860 760 40 8 860 820 20 9 860 830 15 10 860 870 40 11 860 840 20 12 860 900 40 13 860 890 30 13-1 860 890 30 14 860 760 40 15 860 760 40 16 860 760 40 17 860 760 40 18 860 760 40 19 800 770 40 20 920 790 40 21 860 920 40 The flat masks after etching described above were found to have no blurred perifery of pierced hole and have sufficient etching performance. Their average austenite grain size, degree of mixed grain for austenite grains, tensile properties (n value, r value, and elongation) and gathering degree of {331}, {210}, and {211} plane were determined after annealing before press-forming. The gathering degree of {331}, {210}, and {211} plane was determined by X-ray diffraction method which was described before. The result is shown in Table 3. The alloy sheet of material No. 4 was subjected to stress relief annealing under the condition described above, annealing before press-forming under the condition given in Table 2, and to etching to form a flat mask followed by press-forming. The characteristics of the material were determined by the same methods applied to other materials. Partial color-phase shift was determined after blackening the press-formed shadow mask, assembling the mask into the cathode ray tube, and irradiating electron beam onto the cathode ray tube for a specified period. Table 4 shows the result of experiments of press-formablity (shape-fix ability, fitness to die, crack generation on alloy sheet, blurred periphery of pierced hole), partial color-phase shift and corrosion resistance (generation of spot rust; number/100 cm 2 ). Material No. Alloy No. Press-formability Partial color phase shift Corrosion resistance Shape fixability Fitness to die Cracking on the alloy sheet Blurred periphery of the pierced hole Generation of spot rust (number/100cm 2 ) 1 1 o ○ ○ No 0 No 4 2 2 o ○ ○ No 0 No 4 3 3 o ○ ○ No 0 No 3 4 4 ○ ○ No 0 No 2 5 5 ○ ○ No 0 No 4 6 6 ○ ○ No 0 No 1 7 7 ○ ○ No 0 No 1 8 8 ○ ○ No 0 No 1 9 9 ○ ○ No 0 No 0 10 10 o ○ ○ No 0 No 3 11 11 o ○ ○ No 0 No 6 12 12 o ○ ○ No 0 No 2 13 13 o ○ ○ No 0 No 4 13-1 13-1 o ○ ○ No 0 No 4 14 14 ○ × No 0 No 4 15 15 × ○ Yes 5 - 3 16 16 ○ × No 0 No 20 17 17 × ○ Yes 3 - 4 18 18 × ○ Yes 7 - 4 19 19 ○ ○ No 0 Yes 3 20 20 ○ ○ No 0 Yes 3 21 21 ○ ○ Yes 4 No 3 As Table 4 clearly indicates, materials No. 1 through No. 13 and material No. 13-1 having chemical composition, gathering degrees of {331}, {210}, and {211} plane, average austenite grain size, and degree of mixed grain for austenite grains within the range specified in the present invention, show high press-formability without inducing partial color-phase shift and have corrosion resistance better than material No. 16 described later. Material No. 4 was etched after annealing before press-forming, and the flat mask prepared from the material induced no blurred periphery of pierced hole and showed satisfactory etching performance. Material No. 13-1 which includes more Co than other materials showed characterlistics as excellent as others. On the contrary, material No. 14 gives Si content of 0.12 wt.% and material No. 16 gives N content of 0.0035 wt.%, which are more than the upper limit of the present invention, and raises problem of fitness to dies. Material No. 15 gives O content of 0.0035 wt.%, which is more than the upper limit of the present invention and also gives average austenite grain size (referred to simply as "average grain size" hereafter) of 13 µm, which is less than the lower limit of this invention, results in a poor shape fix ability, induces crack generation on alloy sheet, gives degree of mixed grain for austenite grains (referred to simply as "degree of mixed grain" hereafter) above the upper limit of the present invention and results in blurred periphery of pierced hole to raise problem of press-formability. Cr is not added to material No. 16, which shows cosrrosion resistance much inferior to those of the examples of the present invention. Material No. 17 and No. 18 give B content of 0.0035 wt.% and 0.0033 wt.%, respectively, which are more than the upper limit of the present invention and give average grain sizes of 12 µm and 14 µm, respectively, which are less than the lower limit of the present invention, 15µm, resulting in poor shape fix ability. Also degrees of mixed grain of material No. 17 and No. 18 are 56% and 63%, respectively, which are more than the upper limit of the present invention, inducing blurred periphery of the pierced hole. The gathering degrees of {211} plane of the materials are 30% and 34%, which are more than the upper limit of the present invention, 20%, inducing cracks on the alloy sheet, and raise problems of press-formability. Also materials No. 19 show the degree of mixed grain of 21%, which are more than the upper limit of the present invention, 20%. Material No. 20 show the gathering degree of {331} plane of 38%, which are more than the upper limit of the present invention, 38%. Both materials give partial color phase shift, causing quality problems of screen. Material No. 21 shows average grain size of 52 µm, which is more than the upper limit of the present invention, 45 µm, to generate crack on the alloy sheet and induces blurred periphery of the pierced hole, which results in press-formability problem. Material No. 21 gives gathering degree of {211} plane of 23%, which is more than the upper limit of the present invention, 20%, and the crystal orientation becomes more intense with the increase of average grain size under the condition of annealing before press-forming, at 920°C for 40min. The above discussion clearly shows that Fe-Ni-Cr alloy sheet and Fe-Ni-Co-Cr alloy sheet for shadow mask having high press-forming quality, screen quality and corrosion resistance are prepared by adjusting the chemical composition, gathering degrees of {331}, {210}, and {211} plane, average grain size, and degree of mixed grain within the range specified in the present invention. Example 2 The hot-rolled strips of alloys No. 1 through No. 13 and No. 13-1 which were used in Example 1 were subjected to annealing of hot-rolled sheet under the temperature condition given in Table 5, cold-rolling at the reduction ratio in Table 5 (if the column of CR 1 is blank, it indicates that single cold-rolling was carried applying the reduction ratio given in CR 2 ; if both columns of CR 1 and CR 2 are filled, it indicate that two cold-rollings were carried applying the reduction ratio given in each column). After cold-rolling, the materials were treated by recrystallization annealing at 810°C for 1min., finish cold-rolling at the reduction ratio of cold-rolling given in Table 5, stress relief annealing at 530°C for 0.5sec., to obtain alloy sheets of materials No. 22 through No. 47 each having 0.25mm of thickness. Alloy sheets of materials No. 22 through No. 39, No. 41, No. 42, and No. 44 through No. 47 were etched and formed to flat masks. The flat masks were then treated by the annealing before press-forming under the condition given in Table 5 and by press-forming to determine the press-forming quality and partial color-phase shift generation, which characteristics are listed in Table 7. The results are shown in Table 6. Material No. Alloy No. Press-formability Partial color phase shift Shape fix ability Fitness to die Cracking on the alloy sheet Blurred periphery of the pierced hole 22 1 ○ ○ No 0 Yes 23 2 ○ ○ Yes 0 No 24 6 ○ ○ No 2 No 25 12 ○ ○ No 3 No 26 4 × ○ Yes 0 - 27 5 ○ ○ Yes 5 Yes 28 5 ○ ○ Yes 5 No 29 9 ○ ○ Yes 0 Yes 30 8 × ○ Yes 0 - 31 1 ○ ○ No 0 No 32 1 ○ ○ No 0 No 33 2 o ○ ○ No 0 No 34 5 ○ ○ No 0 No 35 4 ○ ○ No 0 No 36 4 o ○ ○ No 0 No 37 3 ○ ○ No 0 No 38 6 o ○ ○ No 0 No 39 9 o ○ ○ No 0 No 40 10 ○ ○ No 0 No 41 7 o ○ ○ No 0 No 42 7 o ○ ○ No 0 No 43 8 ○ ○ No 0 No 44 11 ○ ○ No 0 No 45 13 o ○ ○ No 0 No 46 12 o ○ ○ No 0 No 47 13-1 o ○ ○ No 0 No Before these experiment, average austenite grain size, degree of mixed grains for austenite grain, gathering degrees of {331}, {210} and {211} planes and mechanical properties (n value, r value, and elongation) were investigated. The results are shown in Table 7. These flat masks after etching described above showed no blurred periphery of the pierced hole and found to satisfy the required etching performance. The alloy sheets of materials No. 40 and No. 43 were subjected to stress relief annealing under the condition described above and to annealing before press-forming under the condition given in Table 5 and to etching, then they were formed to flat masks and were press-formed. The characteristics of these materials were determined by the same method applied to other materials. As indicated in Table 6 and Table 7, materials No. 31 through No. 47 have chemical composition within the range specified in the present invention and have the conditions of annealing of hot-rolled sheet, reduction ratio of first and second cold-rolling, finish cold-rolling, annealing before press-forming (temperature: T, °C, time: t, min.), gathering degree of {331}, {210}, and {211} plane, average grain size, and degree of mixed grain within the range specified in the present invention. So the materials No. 31 through No. 47 show high press-formability without inducing partial color-phase shift. Materials No. 40 and No. 43 were etched after the annealing before press-forming. The flat masks prepared from the materials No. 40 and No. 43 showed no blurred periphery of the piercedhole and gave sufficient etching performance. Material No. 47 including Co also shows the excellent characteristics. Among the above described materials, materials No. 32, No. 35 through No. 37, No. 39, No. 43 through No. 45 and No. 47 were treated by two cold-rollings where the reduction ratio of first cold-rolling, CR 1 , was 40 to 55%, and they gave less (more preferable) degree of mixed grain than that of the materials of single cold-rolling (materials No. 31, No. 33, No. 34, No. 38, No. 40 through No. 42, No. 46). On the contrary, materials No. 22 was subjected to annealing of hot-rolled sheet at 800°C, which is less than the lower limit of the present invention, 810°C, material No. 23 was subjected to annealing of hot-rolled sheet at 900°C, which is more than the upper limit of the present invention. Both materials have the gathering degrees of {210} and {211} plane more than the upper limit of the present invention. Material No. 22 gives partial color-phase shift which causes a problem of screen quality, and Material 23 gives cracking on the alloy sheet which gives a problem of press-formability. Material No. 24 is subjected to one cold-rolling at the reduction ratio of 95%, which is more than the upper limit of the present invention, 94%, and material No. 25 was subjected to one cold-rolling at the reduction ratio of 80%, which is less than the lower limit of the present invention, 81%. Both materials give degrees of mixed grain of 59% and 55%, respectively, which are more than the upper limit of the present invention, that induce blurred periphery of the pierced hole to raise problem of press-formability. Material No. 26 was subjected to the one cold-rolling at the reduction ratio of 40%, which is more than the upper limit of the present invention, 29%, and material No. 27 was subjected to the one cold-rolling at the reduction ratio of 12%, which is less than the lower limit of the present invention, 14%. Material No. 26 gives average grain size of 13 µm, which is less than the lower limit of the present invention, 15µm, inducing a problem of the shape fix ability to generate cracking on the alloy sheet. Material No. 27 gives a degree of mixed grain of 60%, which is more than the upper limit of the present invention, 50%, inducing blurred periphery of pierced hole. In addition, Material No. 27 gives the gathering degree of {211} plane of 23%, which is more than the upper limit of the present invention, 20%, inducing cracking on the alloy sheet, and gives gathering degree of {210} plane, which is more than the upper limit of the present invention, inducing partial color-phase shift. Maerial No. 28 was subjected to the annealing before press-forming at 920°C, which is more than the upper limit of the present invention, 900°C, material No. 29 was subjected to the annealing before press-forming for 50 min., which is more than the upper limit of the present invention, 40 min., and as for material No. 30, annealing temperature (T) does not satisfy the equation of(T ≥ - 123logt + 937) . Material No. 28 gives average grain size of 48 µm, which is more than the upper limit of the present invention, 45µm, inducing a problem of blurred periphery of the pierced hole. Material No. 28 also gives the gathering degree of {211} plane of 25%, which is more than the upper limit of the present invention, 20%, inducing cracking on the alloy sheet. Material No. 29 gives the gathering degree of {331} plane of 38%, which is more than the upper limit of the present invention, 35%, inducing cracking on the alloy sheet and partial color phase shift. Material No. 30 gives average grain size of 13µm, which is less than the lower limit of the present invention, 15µmm, inducing a problem of shape fix ability. Material No. 30 also gives the gathering degree of {211} plane of 26%, which is more than the upper limit of the present invention, 20%, inducing cracking on the alloy sheet. As described above in detail, control of the conditions of annealing of hot-rolled sheet, cold-rolling, reduction ratio of finish cold-rolling, and annealing before press-forming within the range specified in the present invention is as important as the chemical composition to be in the range specified in the present invention to provide the press-formability and screen quality intended by the present invention. As seen in materials No. 4, No. 40, and No. 43 used in the embodiment, the flat masks obtained from Fe-Ni-Cr and Fe-Ni-Co-Cr alloy sheets having press-formability required by the present invention without generating partial color-phase shift show no blurred periphery of the pierced hole and give sufficient etching performance. As clarified in Example 1 and Example 2, the case that the gathering degree of {211} plane exceeds 20% and/or that average grain size is outside the scope of this invention provides low value of elongation, n value, and r value after annealing before press-forming compared with the preferred embodiment of the present invention. In addition, when the gathering degree of {211} plane increases and average grain size is out of specific range, or when at least one of the condition is satisfied, these values are presumably decreased inducing crack generation during press-forming.

Claims: Claims for the following Contracting States : DE, FR An alloy sheet consisting of 34 to 38 wt.% Ni, more than 0.05 to 3 wt.% Cr, 0.001 to 0.1 wt.% Si, 0.0001 to 0.003 wt.% O, 0.0001 to 0.003 wt.% N, 1 wt.% or less Co, 0.003 wt.% or less B, and optionally 0,0001 to 0,004 wt.% C, 0,001 to 0,35 wt.%Mn, 2 ppm or less H and the balance being Fe; said alloy sheet having an average austenite grain size of 15 to 45 µm and a degree of mixed grain for austenite grains of 50% or less, said degree of mixed grain being expressed by an equation of (10.5 Dmax - DID) x 100 (%), where D is an average austenite grain size, Dmax is a maximum austenite grain size in said alloy sheet, and 10.5Dmax - DI means an absoute value of (0.5Dmax -D); and a gathering degree of {331} plane on a surface of said alloy sheet being 35% or less, that of {210} plane 20% or less and that of {211} plane 20% or less. The alloy sheet of claim 1, wherein said Ni content is 35 to 37 wt.%. The alloy sheet of claim 1, wherein said Co content is 0.001 to 1 wt.%. An alloy sheet consisting of 27 to 38 wt.% Ni, 0.05 to 3 wt.% Cr, more than 1 to less than 7 wt.% Co, 0.001 to 0.1 wt.% Si, 0.0001 to 0.003 wt.% O, 0.0001 to 0.003 wt.% N, 0.003 wt.% or less B, and optionally 0,0001 to 0,004 wt.% C, 0,001 to 0,35 wt.%Mn, 2 ppm or less H and the balance being Fe; said alloy sheet having an average austenite grain size of 15 to 45 µm and a degree of mixed grain for austenite grains of 50 or less, said degree of mixed grain being expressed by an equation of (|0.5 Dmax - D|/D) x 100 (%), where D is an average austenite grain size, Dmax is a maximum austenite grain size in said alloy sheet, and |0.5Dmax - D| means an absolute value of (0.5Dmax -D); and a gathering degree of {331} plane on a surface of said alloy sheet being 35% or less, that of {210} plane 20% or less and that of {211} plane 20% or less. The alloy sheet of claim 4, wherein said Ni content is 30 to 33 wt.%. The alloy sheet of claim 4, wherein said Co content is 3 to 6 wt.%. An alloy sheet accoding to claim or 4, wherein said gathering degree of {210} plane is 16% or less. An alloy sheet according to claim 1 or 4, wherein the degree of said mixed grain for austenite grain size is 40% or less, said degree of mixed grain being expressed by the eqution of (|0.5 Dmax - D|/D) x 100 (%), where D is an average austenite grain size, and Dmax is a maximum austenite grain size in said alloy sheet. A method for manufacturing an alloy sheet according to any one of claims 1-8 comprising the steps of: (a) hot-rolling a slab containing Fe, Ni and Cr into a hot-rolled strip; (b) annealing said hot-rolled strip in a temperature range of 810 to 890 °C; (c) cold-rolling said annealed hot-rolled strip at a reduction ratio of 81 to 94% into a cold-rolled sheet; (d) a recrystallization annealing step of annealing said cold-rolled sheet (e) finish cold-rolling said cold-rolled sheet subjected to the recrystallization annealing at a reduction ratio of 14 to 29%; (f) a stress relief annealing step of annealing said cold-rolled sheet subjected to the finish cold-rolling; and (g) annealing, before press-forming, the cold-rolled sheet subjected to the stress relief annealing in a temperature range of 740 to 900°C for 2 to 40 min. and on conditions satisfying an equation below; T ≥-123logt + 937, where T is a temperature ( °C) and t is a time (min.) for the annealing. The method of claim 9, wherein the reduction ratio of said cold-rolling is 84 to 92%. The method of claim 9, wherein said recrystallization annealing is performed in a temperature range of 810 to 840°C for 0.5 to 3 min. The method of claim 9, wherein said stress relief annealing is performed in a temperature range of 450 to 540°C for 0.5 to 300 sec. A method for manufacturing an alloy sheet according to any one of claims 1-8 comprising the steps of: (a) hot-rolling a slab contianing Fe, Ni and Cr into a hot-rolled strip; (b) annealing said hot-rolled strip in a temperature range of 810 to 890 °C; (c) a first cold-rolling step of cold-rolling said annealed hot-rolled strip at a reduction ratio of 40 to 55% into a cold-rolled sheet; (d) a first recrystallization annealing step of annealing said cold-rolled sheet; (e) a second cold-rolling step of cold rolling said annealed cold-rolled sheet at a reduction ratio of 81 to 94%; (f) a second recrystallization annealing step of annealing said cold-rolled sheet; (g) finish cold-rolling said sheet subjected to the second recrystallization annealing at a reduction ratio of 14 to 29%; (h) a stress relief annealing step of annealing said finish-cold rolled sheet; (i) annealing, before press-forming, the cold-rolled sheet subjected to the stress relief annealing in a temperature range of 740 to 900°C for 2 to 40 min. and on conditions satisfying the equation below; T ≥-123logt + 937, where T is a temperature ( °C) and t is a time (min.) for the annealing. Thc method of claim 13, wherein said first recrystallization annealing is performed in a temperature range of 810 to 840°C for 0.5 to 3 min. The method of claim 13, wherein said second recrystallization annealing is performed in a temperature range of 810 to 840°C for 0.5 to 3 min. The method of claim 13, wherein said stress relief annealing is performed in a temperature range of 450 to 510°C for 0.5 to 3 min. The method according to claims 9 or 13, wherein said slab consists of 34 to 38 wt.% Ni, more than 0.05 to 3 wt.% Cr, 0.001 to 0.1 wt.% Si, 0.0001 to 0.003 wt.% O, 0.0001 to 0.003 wt.% N, 1 wt.% or less Co, 0.003 wt.% or less B, and the balance being Fe. The method according to claims 9 or 13, wherein said slab consists of 27 to 38 wt.% Ni, 0.05 to 3 wt.% Cr, more than 1 and less than 7 wt.% Co, 0.001 to 0.1 wt.% Si, 0.0001 to 0.003 wt.% O, 0.0001 to 0.003 wt.% N, 0.003 wt.% or less B, and the balance being Fe. Claims for the following Contracting State : GB An alloy sheet consisting of 34 to 38 wt.% Ni, 0.05 to 3 wt.% Cr, 0.001 to 0.1 wt.% Si, 0.0001 to 0.003 wt.% O, 0.0001 to 0.003 wt.% N, 1 wt.% or less Co, 0.003 wt.% or less B, and optionally 0,0001 to 0,004 wt.% C, 0,001 to 0,35 wt.%Mn, 2 ppm or less H and the balance being Fe; said alloy sheet having an average austenite grain size of 15 to 45 µm and a degree of mixed grain for austenite grains of 50% or less, said degree of mixed grain being expressed by an equation of (|0.5 Dmax - D|/D) x 100 (%), where D is an average austenite grain size, Dmax is a maximum austenite grain size in said alloy sheet, and |0.5Dmax - D| means an absoute value of (0.5Dmax -D); and a gathering degree of {331} plane on a surface of said alloy sheet being 35% or less, that of {210} plane 20% or less and that of {211} plane 20% or less. The alloy sheet of claim 1, wherein said Ni content is 35 to 37 wt.%. The alloy sheet of claim 1, wherein said Co content is 0.001 to 1 wt.%. An alloy sheet consisting of 27 to 38 wt.% Ni, 0.05 to 3 wt.% Cr, more than 1 to less than 7 wt.% Co, 0.001 to 0.1 wt.% Si, 0.0001 to 0.003 wt.% O, 0.0001 to 0.003 wt.% N, 0.003 wt.% or less B, and optionally 0,0001 to 0,004 wt.% C, 0,001 to 0,35 wt.%Mn, 2 ppm or less H and the balance being Fe; said alloy sheet having an average austenite grain size of 15 to 45 µm and a degree of mixed grain for austenite grains of 50% or less, said degree of mixed grain being expressed by an equation of (|0.5 Dmax - D|/D) x 100 (%), where D is an average austenite grain size, Dmax is a maximum austenite grain size in said alloy sheet, and |0.5Dmax - D| means an absolute value of (0.5Dmax-D); and a gathering degree of {331} plane on a surface of said alloy sheet being 35% or less, that of {210} plane 20% or less and that of {211} plane 20% or less. The alloy sheet of claim 4, wherein said Ni content is 30 to 33 wt.%. The alloy sheet of claim 4, wherein said Co content is 3 to 6 wt.%. An alloy sheet accoding to claim 1 or 4, wherein said gathering degree of {210} plane is 16% or less. An alloy sheet according to claim 1 or 4, wherein the degree of said mixed grain for austenite grain size is 40% or less, said degree of mixed grain being expressed by the eqution of (|0.5 Dmax - D|/D) x 100 (%), where D is an average austenite grain size, and Dmax is a maximum austenite grain size in said alloy sheet. A method for manufacturing an alloy sheet according to any one of claims 1-8, comprising the steps of: (a) hot-rolling a slab containing Fe, Ni and Cr into a hot-rolled strip; (b) annealing said hot-rolled strip in a temperature range of 810 to 890 °C; (c) cold-rolling said annealed hot-rolled strip at a reduction ratio of 81 to 94% into a cold-rolled sheet; (d) a recrystallization annealing step of annealing said cold-rolled sheet (e) finish cold-rolling said cold-rolled sheet subjected to the recrystallization annealing at a reduction ratio of 14 to 29%; (f) a stress relief annealing step of annealing said cold-rolled sheet subjected to the finish cold-rolling; and (g) annealing, before press-forming, the cold-rolled sheet subjected to the stress relief annealing in a temperature range of 740 to 900°C for 2 to 40 min. and on conditions satisfying an equation below; T ≥-123logt + 937, where T is a temperature ( °C) and t is a time (min.) for the annealing. The method of claim 9, wherein the reduction ratio of said cold-rolling is 84 to 92%. The method of claim 9, wherein said recrystallization annealing is performed in a temperature range of 810 to 840°C for 0.5 to 3 min. The method of claim 9, wherein said stress relief annealing is performed in a temperature range of 450 to 540°C for 0.5 to 300 sec. A method for manufacturing an alloy sheet according to any one of claims 1-8 comprising the steps of: (a) hot-rolling a slab contianing Fe, Ni and Cr into a hot-rolled strip; (b) annealing said hot-rolled strip in a temperature range of 810 to 890 °C; (c) a first cold-rolling step of cold-rolling said annealed hot-rolled strip at a reduction ratio of 40 to 55% into a cold-rolled sheet; (d) a first recrystallization annealing step of annealing said cold-rolled sheet; (e) a second cold-rolling step of cold rolling said annealed cold-rolled sheet at a reduction ratio of 81 to 94%; (f) a second recrystallization annealing step of annealing said cold-rolled sheet; (g) finish cold-rolling said sheet subjected to the second recrystallization annealing at a reduction ratio of 14 to 29%; (h) a stress relief annealing step of annealing said finish-cold rolled sheet; (i) annealing, before press-forming, the cold-rolled sheet subjected to the stress relief annealing in a temperature range of 740 to 900°C for 2 to 40 min. and on conditions satisfying the equation below; T ≥-123logt + 937, where T is a temperature ( °C) and t is a time (min.) for the annealing. The method of claim 13, wherein said first recrystallization annealing is performed in a temperature range of 810 to 840°C for 0.5 to 3 min. The method of claim 13, wherein said second recrystallization annealing is performed in a temperature range of 810 to 840°C for 0.5 to 3 min. The method of claim 13, wherein said stress relief annealing is performed in a temperature range of 450 to 510°C for 0.5 to 3 min. The method according to claims 9 or 13, wherein said slab consists of 34 to 38 wt.% Ni, 0.05 to 3 wt.% Cr, 0.001 to 0.1 wt.% Si, 0.0001 to 0.003 wt.% O, 0.0001 to 0.003 wt.% N, 1 wt.% or less Co, 0.003 wt.% or less B, and the balance being Fe. The method according to claims 9 or 13, wherein said slab consists of 27 to 38 wt.% Ni, 0.05 to 3 wt.% Cr, more than 1 and less than 7 wt.% Co, 0.001 to 0.1 wt.% Si, 0.0001 to 0.003 wt.% O, 0.0001 to 0.003 wt.% N, 0.003 wt.% or less B, and the balance being Fe.

Inventors: HIASA MICHIHITO, INOUE TADASHI, TSURU KIYOSHI, YAMAUCHI KATSUHISA, HIASA, MICHIHITO, INOUE, TADASHI, TSURU, KIYOSHI, YAMAUCHI, KATSUHISA, HIASA, MICHIHITO, C/O INTELLECTUAL PROPERTY DEPT., INOUE, TADASHI, C/O INTELLECTUAL PROPERTY DEPT., TSURU, KIYOSHI, C/O INTELLECTUAL PROPERTY DEPT., YAMAUCHI, KATSUHISA, C/O INTELLECTUAL PTY DEPT.
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1994
Main classification: C22C38
IPC codes: C22C38, C21D8, H01J9, C21D9, H01J29